Ziad,

I made the same mistake and finally figured out that the shape of the hyperbolas in your two plots is determined by the fact that your w0 is negative (just rearrange your formulas into the h = sign(w0 + w1*x1^2 + w2*x2^2) form to see this). The "inner" part of your hyperbolas assumes the value -1 instead of +1 as required. If your plot answer [d] with a positive w0, which makes the inner region assume the value +1, then the the orientation of the hyperbolas flips to the required shape:

http://www.wolframalpha.com/input/?i...Bx2%5E2+%3D-+1
Dirk